Single Photon Emission Computed Tomography (SPECT) reconstructs three dimensional images of radioactive source distributions in the body using a sequence of planar images acquired over a range of angles around the patient.
In order to reconstruct the tomographic image from the set of planar images it is necessary to know the direction from which the photons detected at a given point in the image originated. For X-ray computed tomography (CT), the direction is defined by a line from the anode to the X-ray detector. For Positron Emission Tomography (PET), it is a line between the pair of detectors in which the two coincident 511 keV photons are detected. In SPECT, the direction is usually defined by a collimator—a lead plate with 20,000 to 50,000 small holes formed in it which restricts the detected incident photons to only those with known angles of incidence at the detector.
The most popular collimator type is the parallel beam collimator, in which the holes are designed to point perpendicular to the detector surface. Another type of collimator is referred to as a fan beam collimator. With the fan beam collimator, the holes in one dimension (transverse) focus to a point; there is no focusing in the axial dimension. Another type of collimator is a cone beam collimator that focuses to a single point in both transverse and axial dimensions. Yet another type of collimator is a multi-focal or variable focal length fan beam collimators that has multiple focal points.
Image reconstruction algorithms in the current state of the art assume that the construction of these collimators is perfect. Such algorithms perform back projection of planar projection data and forward projections of the object estimates under this assumption. In reality, however, collimators are not perfect. Their construction is subject to dimensional errors such that all holes do not point in the ideal intended direction. This leads to errors in forward and backward projections and, results in distortions and degradation of the resolution in the final tomographic images.
More recently, a number of methods are being developed by those in the art to address the hole pointing errors of the collimators and account for the inaccuracies in the collimator hole pointing directions into the image reconstruction process to remove distortions and improve the image resolution. One of the methods that the inventors are aware of is a method in which the collimator holes' actual orientation or pointing directions are measured using a multiple parallel line radiation sources and incorporate that information into the image reconstruction process. The measurement method involves imaging a set of multiple parallel line radiation sources once through the collimator being measured and another time through a parallel hole reference collimator generating two sets of line images of the multiple line radiation sources. The lines in the line images represent the radiation intensity profiles of the radiation emitted by the multiple parallel line radiation sources. One set of the line images is associated with the collimator being measured and represents the intensity profiles of the radiation emitted by the multiple parallel line radiation sources collimated through the collimator being measured. The second set of the line images is associated with the reference collimator and represents the intensity profiles of the radiation emitted by the multiple parallel line radiation sources collimated through the reference collimator. Because of the geometries involved in the arrangement utilized in the method, the calculation of the collimator holes' orientation angles require pairing or matching of the lines in one set of line images to their corresponding lines in the second set of line images. An important aspect of the method is properly identifying and pairing the lines between the two sets of line images.